flu and so many other germs, we're told again and again that hand sanitizers and soap are the best defense. last year, americans spent more than $170 million on hand sanitizers alone. so, we wondered, what works best? and how do the sanitizers compare in a showdown with soap? well, tonight, our chief medical editor dr. richard besser put them to the test and gets some real answers. >> reporter: clean? as many as 400,000 germs per hand. that's how many bacteria travel with us every day. so, here at the university of maryland food safety lab, six brave grad students and i signed up for a nasty experiment -- voluntarily dousing our hands in a toxic brew of millions of e-coli bacteria. yep, e-coli. good? something a little odd about rubbing e. coli. to see what would take it off. first up, hand sanitizers. we tested two kinds you'll find at the store. one with 60% alcohol and one so-called natural formula containing no alcohol. the key with hand sanitizers is to use enough of it. your hands should take a full 15 seconds or longer to dry. now let that air dry, okay? then, we pressed our hands o